Low temperature cofired ceramic (LTCC) substrates used in connection with electronic circuitry, and more particularly integrated circuits, are generally well known and typically comprise a multilayered structure of ceramic material which includes stripline circuit patterns and openings therethrough called vias for providing circuit interconnection between layers. The layers are laminated together by externally applied pressure from, for example, a hydrostatic press. Such a press may apply pressure on the order of 250 to 6000 psi or higher to the laminate components.
Many such substrates have cavities in their surfaces. To apply pressure evenly to the cavities, silicone molds having a shape corresponding to the cavities may be placed in the cavities. As the cavities have become more numerous and complex in shape, having, for example, internal ledges and other non-uniform features, the silicone molds have become more complex as well and therefore more expensive.
One method for forming LTCC substrates without using silicone molds is disclosed in U.S. Pat. No. 5,683,535, entitled “Method and Apparatus of Producing Cavities in LTCC Substrates,” the entire contents of which are hereby incorporated by reference. A method similar to the one described in that patent is outlined below.
FIG. 1 illustrates a stack of a plurality of green, unfired, LTCC sheets 12 on a stainless steel tooling plate 14 having one or more alignment pins 16 which receive corresponding holes 18 in the LTCC sheets 12. The stack includes a plurality of cavities 24. A 0.025 in. thick polyimide template 20 having openings 22 corresponding to the cavities is placed over the stack 12. The cavities 24 may include various internal ledges and internal features such as ledges 25.
In order to protect the corners of the cavities 24 and to reduce rounding of the edges thereof during the lamination process, a single sheet 36 of 0.008 in. thick talc coated latex rubber is placed over the template. Next, pairs of 0.125 in. thick silicone foam sheets 28 and 30 are trimmed to the required area of the parts and are placed above and below the assembly including the tooling plate 14, template 20, and silicone mold 26. The assembly is then placed in a flexible vacuum bag, shown schematically by reference numeral 32. The silicone rubber sheets 28 and 30 act as a protective barrier to keep the vacuum bag 32 from being punctured by a rigid edge of the lamination stack-up assembly located therebetween.
The vacuum bag 32 and its contents are then placed in a hydrostatic pressure chamber shown by the phantom lines 34. The hydrostatic pressure chamber 34 generates a pressure of between about 250 psi and 6000 psi which isostatically laminates the LTCC sheets 12 together, forming a composite substrate structure.
Metallic wires, ribbons and similar elements are sometimes attached to a substrate after the lamination process. The bonds between these elements and the substrate must be sufficiently strong to prevent separation during normal use. It has been found, however, that the bondablity of the cavity interiors is generally less than the bondability of the substrate surfaces surrounding the cavities. This difference is particularly noticeable in substrates made according to the process disclosed in U.S. Pat. No. 5,683,535. It would therefore be desirable to provide a method of producing an LTCC substrate having cavities that avoids the need for separate molds in each cavity while at the same time producing cavities having acceptable levels of bondability.